Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a semiconductor wafer. The radiation-patterning tool can be, for example, a photomask or a reticle, with the term "photomask" being sometimes understood to refer to masks which define a pattern for an entirety of a wafer, and the term "reticle" being sometimes understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms "photomask" (or more generally "mask") and "reticle" are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms "photomask" and "reticle" will be given their historical distinction such that the term "photomask" will refer to a patterning tool that defines a pattern for an entirety of a wafer, and the term "reticle" will refer to a patterning tool that defines a pattern for only a portion of a wafer.
Radiation-patterning tools contain light-restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light-transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. The wafer is provided with a layer of photosensitive resist material commonly referred to as photoresist. Radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers the mask pattern to the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials on the wafer proximate the photoresist.
A method of forming a radiation-patterning tool is to provide a layer of light-restrictive material (such as, for example, chrome) over a light-transmissive substrate (such as, for example, a fused silicon such as quartz), and subsequently etch a pattern into the light-restrictive material. The pattern can be etched by, for example, providing a masking material over the light-restrictive material, forming a pattern in the masking layer with an electron beam or a laser beam, and transferring the pattern to the underlying light-restrictive material with an etchant that removes exposed portions of the light-restrictive material. The patterned light-restrictive material can be considered to be "supported by" the tool, as well as to be "on" or "in" the tool.
In a typical process of fabricating semiconductor circuitry, a desired circuit pattern will be designed, and subsequently a radiation-patterning tool will be formed to create the pattern. A problem in forming the radiation-patterning tool is in correlating particular pattern shapes desired in the integrated circuitry to pattern shapes utilized in the tool. Specifically, a pattern shape formed in a tool will typically not be identical to a pattern shape generated with the tool because of interference patterns formed from light passing through the tool. The problem is described in FIGS. 1-3.
FIG. 1 illustrates a portion of a semiconductor wafer 10 comprising a material 12 thereover. Material 12 can comprise, for example, photoresist, and has a desired pattern 14 defined therein. Ultimately, pattern 14 is to be formed by passing light through a radiation-patterning tool to selectively expose the region encompassed by pattern 14 while not exposing other regions of material 12. Accordingly, a radiation-patterning tool is to be constructed which patterns light in the shape of pattern 14. FIGS. 2 and 3 describe alternative approaches for designing such radiation-patterning tools.
Referring to FIG. 2, such illustrates a result obtained from utilizing a radiation-patterning tool having a pattern identical to the shape of pattern 14 formed therein. Specifically, FIG. 2 shows a portion of a radiation patterning tool 16 having a light-restrictive material 18 formed over a substrate (not shown), and a pattern 20 formed within material 18. Pattern 20 constitutes a region wherein light-restrictive material 18 has been removed. FIG. 2 also shows a pattern resulting from passing light through patterning tools 16. Specifically, FIG. 2 shows semiconductive substrate 10 having material 12 thereover, and a pattern 22 corresponding to a region of material 12 exposed to light passing through pattern 20 of tool 16. A dashed line 14 over fragment 10 of FIG. 2 corresponds to the desired pattern shape 14 of FIG. 1. It is noted that pattern 22 is a poor approximation of the desired shape 14, and specifically that the corners of shape 14 are not present, and instead replaced by rounded features in the shape of pattern 22. In referring to FIG. 2, it is to be understood that the shape of pattern 22 is a qualitative approximation to a pattern expected from the shape 20 of tool 16, and is provided for diagrammatic purposes only. The illustrated shape of pattern 22 is not a quantitative representation.
FIG. 3 describes a prior art method which has been developed to compensate for the problem described with reference to FIG. 2. Specifically, FIG. 3 illustrates a radiation-patterning tool 26 having light-restrictive material 18 formed over a substrate (not shown) and a pattern 28 formed therein. Pattern 28 has been developed utilizing optical proximity correction (OPC) software, such as, for example, a Taurus-OPC.TM. module (available from Avant! Corporation of Portland, Oregon). Specifically, the desired pattern 14 (FIG. 1) is digitally mapped and provided to the software program, together with the wavelength of light which is to be passed through a radiation-patterning tool to form the pattern 14. The software then determines a pattern 28 which should be formed in the radiation-patterning tool to pattern the light in a shape which closely approximates the desired shape 14. FIG. 3 illustrates a portion of a semiconductive wafer having material 12 formed thereon and a pattern 30 formed by passing radiation through tool 26. FIG. 3 also shows a dashed line on fragment 10 corresponding to the desired shape 14. It is noted that pattern 30 more closely approximates desired shape 14 than did pattern 22 of FIG. 2. In referring to FIG. 3, it is to be understood that the patterns 28 and 30 are qualitative approximations to actual patterns. The illustrated patterns 28 and 30 are not quantitative representations.
A difficulty in utilizing OPC software can be in reducing the calculation time required for determining corrections for patterning tools having substantial size or complexity. For instance, in dynamic random access memory (DRAM) fabrication, there can be literally millions of circuit elements which are to be patterned with a single radiation-patterning tool. Mapping these elements into OPC software, and subsequently processing the elements to determine appropriate optical proximity corrections can take days. Accordingly, shortcuts have been developed for utilizing OPC in fabrication of DRAM circuitry. For instance, it is recognized that DRAM circuitry frequently comprises highly repetitive regions corresponding to DRAM arrays, and relatively non-repetitive regions corresponding to peripheral circuitry around the arrays. Accordingly, OPC of DRAM arrays is typically done in two distinct steps. One of the steps is to digitize the peripheral circuitry and perform OPC with appropriate software, while ignoring the repetitive regions of the DRAM array. The other of the steps is to first map a single repeated unit of the memory array and perform OPC on such unit. Next, the corrected unit is manually reproduced (stepped) across an entire expanse of a DRAM array to effectively perform OPC on the entirety of the DRAM array while ignoring the peripheral regions. The corrections for the memory array and the peripheral regions are combined to form a pattern which is to be provided on a radiation-patterning tool for creating DRAM circuitry.
It would be desirable to develop improved methods for performing OPC, and particularly to develop improved methods for performing OPC relative to patterns which are to be utilized in forming memory array regions and peripheral regions of DRAM circuitry.